May 2019

44 n LINEAR MOTION May 2019 www.drivesncontrols.com into linear motion through ballscrews. Choosing ballscrews that can handle up to a million cycles during their lifetime requires advanced optimisation calculations that balance load and speed by upgrading the screw-and-nut assembly. These calculations can get quite complex but essentially involve increasing the load capabilities of the assembly by using larger-diameter ballscrews and nuts with twice as many balls. Motor designs also affect actuator lives. Most actuators on electrified vehicles use traditional brush-based DC motors. But these brushes wear out too soon for the extended use that transit designers are now projecting. A better alternative is to replace the brushes with an electromagnetic field, which removes the brushes entirely. There will, of course, be some wear on the bearings and other parts of the motor, but the longevity no longer depends on the brush life. A brushless DC motor also needs a slightly different user interface and will help to control braking in pantograph operation. In a pantograph, the actuator is working against a spring. In one direction, it is the opposing force; in the other direction, it is an assisting force. An opposing spring provides stable, more controlled speed in one direction. An assisting spring, on the other hand, allows a more free-wheeling operation that must be controlled. Typically, the speed is controlled using a friction brake when the spring is assisting. During extended use, the friction brake will wear and limit the life of the actuator. New actuators have been developed to cope with the increasing demands of transport applications. Along with upgraded motors, they have upgraded ballscrew designs, electromagnetic load- holding brakes, and integrated motor controls. The brushless motors are used to control the speed, with an electromechanical brake holding the load Linear actuators have many potential applications in public transport vehicles including door operation and wheelchair access, as well as connecting to overhead power lines (Image: Thomson Industries) in place when the actuator stops. Actuators used in public transit applications are already subject to health and safety regulations, but their increased use will bring improvements here as well. While vehicles in less-frequent operation might currently use a lower protection level such as IP65, greater use of vehicles moving at 100km/h would result in increased exposure to environmental forces and would probably require a higher level of protection. Similarly, seaside applications would have greater exposure to corrosive salt air, requiring protection to IP66 or even IP69K. These factors, combined with stringent public safety standards preventing use of toxic materials, mean that actuator suppliers need to ensure that the materials and sealing strategies they choose will provide safe performance for vehicles with lifetimes of 20 or 30 years. The road ahead In today’s transport applications, actuators primarily provide switching functions, and will continue to do so. But switching requires onboard electronics, enabling actuators to communicate with each other and with other devices, and providing information such as feedback on the position of the pantograph. Actuators will need to evolve to provide such functions. Although actuators used for charging stations are among the first components that need to be upgraded to accommodate the growing numbers of passengers and expanded electrification, other actuator applications may soon follow, including door operation, step levelling, railcar connecting, and gap control for safe and easy passenger access. To date, these applications have been handled mainly by pneumatic or hydraulic actuators, but with increased cycles and the requirement for more efficient and robust components, engineers will need to consider electromechanical actuators for their designs. n